There are a number of devices that use oscillators, and, in particular, crystal oscillators. A crystal oscillator is a circuit that uses the resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. A very precise frequency provides a stable clock signal in digital integrated circuits used in applications such as to keep track of time, to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal. Crystal oscillators are used in devices such as dataloggers, wristwatches, clocks, radios, computers, signal generators, and oscilloscopes.
A tuning-fork quartz crystal oscillator is one of a family of devices that vibrate at a given frequency when invested with energy by way of an electric field. However, internal mechanical stresses coupled with thermal expansion of the device and contraction cause this frequency to vary with temperature. The variation can be roughly characterized by a parabolic function, f(T), such that f(T)=k1T2+k2T+k3. Each crystal is designed with a stability temperature, T0, and corresponding frequency f0, near which small changes in temperature result in small changes in frequency. Most commercially available crystals have a T0 around room temperature (20±2° C.). Operation at temperatures far from T0 results in increasing deviations from f0. When such a crystal is used to generate the sampling clock for an Analog to Digital Converter (ADC) at temperatures far from T0, the sampling rate is inaccurate by the same factor.
Existing methods used to minimize the temperature dependence of the oscillator include (a) heating it to T0, known as furnacing the crystal; (b) cutting the crystal such that its T0 is at the target temperature for operation; or (c) cutting the crystal to create a flatter f(T) function—that is, minimizing k1 and k2. Disadvantages of these methods are that furnacing is power intensive, while custom crystal cutting for flat f(T) or target T0 is cost prohibitive for most applications, and does not compensate for additional temperature variations.
As an example, a data acquisition platform used by the Marine Autonomous Recording Unit (MARU) typically operates near 0° C. for months at a time, resulting in the accumulation of several minutes of sampling period drift. Additionally its power and cost budgets are limited, making furnaced or custom-cut crystals infeasible. As another example, clock synchronization error among multiple data recorders, each with its own quartz crystal oscillator (QCO), can be influenced by temperature variations and these errors accumulate over long duration recordings. This is due to each QCO having its own unique temperature dependence function.
What is needed is a system and methods to minimize the frequency error or clock synchronization error of oscillators. The present invention satisfies this demand.